Channelrhodopsins respond to light because they contain a chromophore molecule (retinal) that can switch between two different shapes. The activation energy, the amount of energy that it takes to switch the chromophore between these two states, is about 240 kJ/mol, which corresponds to the energy of a blue photon (480nm). If we wanted to engineer a rhodopsin to be sensitive to radio frequencies, what activation energy would we want our chromophore to have? Well, let's take the frequency of one of my favorite radio stations 106.7 MHz (let's round to 100 MHz), which would correspond to a wavelength of about 3 m, and an energy of about 0.04 kJ/mol. Therefore, the activation energy required to change the shape of the chromophore molecule would have to be about 0.04 kJ/mol.
Do you see the problem? At human body temperature (37oC, 310K), the amount of thermal energy available is about 2.6 kJ/mol. This means that thermal energy alone will be enough to activate our hypothetical radio-sensitive rhodopsin! The protein would not actually be able to sense radio waves because it would always be on regardless of whether or not radio waves were present.
If you look at calculations like these, you'll see that it is not an accident that animal vision is limited to a small range of the EM spectrum ranging from the near IR to the near UV. It is determined, rather, by the laws of physics. At frequencies significantly below the visible region, you get to the point where thermal energy becomes more energetic than the photons and a chromophore would not be able to distinguish thermal energy from the absorption of photons. At frequencies significantly above the visible region, you get into the range of ionizing radiation, photons so energetic that their energies are comparable to the activation energies for breaking chemical bonds.